Distance protective relay using a programmable thermal model for thermal protection

ABSTRACT

The system includes a distance protective relay for power lines which includes a logic capability which is responsive to settings entered into the relay by an end user to implement the value of those settings into stored thermal model equations which emulate the temperature of the power line conductor. The logic within the relay is organized and has the capability of receiving the setting values entered by the user and to use those in the logic equations to determine the temperature of the conductor.

TECHNICAL FIELD

This invention relates generally to power line thermal protection, andmore specifically concerns such thermal protection implemented with afirst order thermal model emulating the temperature of the power line.

BACKGROUND OF THE INVENTION

Thermal protection for power lines is accomplished by monitoring thetemperature of the wire conductor and generating a trip signal whichopens a circuit breaker when the conductor temperature becomes greaterthan the maximum allowed temperature for the conductor. The temperatureof the conductor is not measured directly, but is obtained by using afirst order thermal model for the conductor. This involves using a heatbalance equation (heat input minus heat losses) of a 1000 foot sectionof conductor.

The 1000 foot section of the line is assumed to have the highesttemperature and to be exposed to the maximum solar radiation. The heatinput portion in the equation is mainly due to heat dissipated in theconductor resistance and the solar heat gain. The heat loss portion isdue primarily to convection and radiation. In general, the equations forestablishing such a thermal model are well known, and the procedures forobtaining the temperatures of overhead conductors using a thermal modelare set forth in IEEE Standard 738-1193, titled “IEEE Standard forCalculating the Current-Temperature Relationship of Bare OverheadConductors”.

A power line thermal model has been used previously in protectiverelays, although the thermal model equations are preestablished and theparameters thereof are determined by the manufacturer. The user has nocontrol over the thermal model or its operation. It is an automaticprocess. This can lead to inaccurate results in some cases. Accordingly,it is desirable that the end user of a protective relay have somecapability to itself establish the thermal model parameters for a moreaccurate and fast determination of power line temperature.

SUMMARY OF THE INVENTION

Accordingly, the present invention is a protective relay system forpower line thermal protection, using programmable logic present withinthe relay, comprising: a protective relay for power lines, whichincludes a programmable logic capability by which the end user of theprotective relay can enter operational settings which are then used bythe relay in carrying out its thermal protection functions; a set ofstored thermal model equations which-emulate the temperature of a powerline conductor, based on a plurality of individual setting values whichare enterable into the relay by the end user, and wherein the logicimplements the entered setting values into the thermal model equationswhich produce an emulated temperature of the conductor; and means forproviding an indication when the temperature of the conductor exceeds apreselected value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a general implementation of a thermalmodel for determining temperature of a power line conductor.

FIG. 2 is a block diagram of a particular example of a thermal modelimplementation.

FIG. 3 is a block diagram of the processing used by a thermal model toproduce a temperature value.

FIG. 4 is an example of a program code for a thermal model.

BEST MODE FOR CARRYING OUT THE INVENTION

As indicated above, use is made of first order thermal models todetermine the temperature of electrical conductors, such as for instancethe temperature in various parts of an induction motor. These principlesare set out in a paper by S. A. Zocholl and Gabriel Benmouyal entitled“Using Thermal Limit Curves to Define Thermal Models for InductionMotors”, Proceedings of the 28th Annual Western Protective RelayConference, Spokane, Wash., October 2001. The first order thermal modelfor protection of a power line is expressed as first order differentialequation: $\begin{matrix}{{{Power}\quad{Supplied}\quad(P)\text{-}{Losses}} = \frac{\mathbb{d}({TC})}{\mathbb{d}t}} & (1)\end{matrix}$The equations below use the following variables:

-   -   P=heat power input to the conductor (kW/kft)    -   THC=conductor heat capacity (kJ/° C. kft)    -   TRA=thermal resistance to ambient (° C. kft/kW)    -   TC=estimated conductor temperature (° C.)    -   TA=ambient temperature (° C.)    -   TI=conductor initial temperature (° C.)    -   I=conductor current (A RMS)    -   rac=AC conductor resistance at 25° C. (Ω/kft)    -   rdelt=temperature coefficient of AC resistance (Ω/° C. kft)    -   Q_(sun)=heat power input from the sun (kW/kft)    -   Q_(radiated)=radiation heat losses (kW/kft)    -   Q_(convected)=convection heat losses (kW/kft)    -   Δt=processing interval or integration step (s)        Basically, the heating power supplied to the conductor can be        expressed as:        p=I ²·(rac)+(TC−25)·rdelt)+Q _(sun)  (2)        Further, the heat losses from the conductor are expressed in the        following equation: $\begin{matrix}        {\frac{{TC} - {TA}}{TRA} = {Q_{radiated} + Q_{convected}}} & (3)        \end{matrix}$        The conductor temperature differential equation is then:        $\begin{matrix}        {{{THC}\frac{\mathbb{d}{TC}}{\mathbb{d}t}} = {{I^{2} \cdot \left( {{rac} + {\left( {{TC} - 25} \right) \cdot {rdelt}}} \right)} + Q_{sun} - \frac{{TC} - {TA}}{TRA}}} & (4)        \end{matrix}$        From Equation (4), the solution for the conductor temperature is        obtained as follows: $\begin{matrix}        {{{TC}(t)} = {{\int_{0}^{t}{\left( {\frac{{I^{2} \cdot \left( {{rac} + {\left( {{TC} - 25} \right) \cdot {rdelt}}} \right)} + Q_{sun}}{THC} - \frac{{TC} - {TA}}{{TRA} \cdot {THC}}} \right){\mathbb{d}T}}} + {TI}}} & (5)        \end{matrix}$

The equation is solvable numerically by computing the conductortemperature in successive time increments ATC, to determine theconductor temperature. $\begin{matrix}{{\Delta\quad{TC}} = {\left( {\frac{{I^{2} \cdot \left( {{rac} + {\left( {{TC} - 25} \right) \cdot {rdelt}}} \right)} + Q_{sun}}{THC} - \frac{{TC} - {TA}}{{TRA} \cdot {THC}}} \right)\Delta\quad T}} & (6)\end{matrix}$

The conductor temperature is determined successively as follows:TC _(new) =ΔTC+TC _(old)  (7)The thermal model is illustrated in FIG. 1, where TTH is the maximumallowable temperature threshold, above which the circuit breaker for thepower line can be tripped, while TTL is an alarm threshold. When Q_(sun)is assumed to be a constant, and when t is infinite, the conductortemperature is represented by the following equation: $\begin{matrix}{I = \sqrt{\frac{{TTH} - {TA} - {Q_{sun} \cdot {TRA}}}{\left( {{rac} + {\left( {{TTH} - 25} \right) \cdot {rdelt}}} \right) \cdot {TRA}}}} & (8)\end{matrix}$

Referring to FIG. 1, the heat power into the conductor is represented bysource 12, while THC (conductor heat thermal capacity) is represented bycapacitor 14, TRA (thermal resistance to the ambient temperature value)is represented by resistance 16 and TA (ambient temperature) isrepresented by battery 18. The result of the thermal model is anestimated conductor temperature TC at output point 20. The temperaturevalue at output point 20 is compared to the value of TTL (input 21) incomparator 22 to produce an alarm, and compared with the value TTH(input 23) in comparator 24 to produce a trip signal.

Several specific values in the above equations must be determined inorder to solve the equations and complete the first order differentialequation. The value of Q_(sun) is determined by the following equation:Q _(sun) =SAC·DIA·SIR  (9)The variables in the determination of Q_(sun) are set forth below.

-   -   d=solar declination (degrees)    -   n=days of the year (days)    -   Hrs=local time in hours (hours    -   LocT=local time (degrees)    -   Ws=solar time (degrees)    -   SAC=solar absorption coefficient (unitless)    -   DIA=conductor diameter (inches)    -   z=solar zenith angle (degrees)    -   LSTD=standard time meridian (degrees west longitude)    -   LON=conductor longitude (degrees west longitude)    -   LAT=conductor latitude (degrees north latitude)    -   SIR=solar incident radiation (kW/in)    -   Q_(sun)=solar heating (kW/kft)

Referring to Equation 9, the value of SIR is typically provided by alook-up table as a function of the solar zenith angle. The solar zenithangle is determined indirectly by its cosine as a function of the solardeclination d, the conductor latitude LAT and the solar time Ws as inthe following equation:cos(z)=sin(d)·sin(LAT)+cos(d)·cos(LAT)·cos(Ws)  (10)The value of the solar incident radiation (SIR) can then be computedusing the following equation:SIR=α ₅·cos(z)⁵+α₄·cos(z)⁴+α₃·cos(z)³+α₂·cos(z)²+α₁·cos(z)¹+α₀  (11)Power n of cos(z) can be determined by multiplying the number n times byitself. The solar time Ws is provided as a function of the local timeLocT, the longitude value LON and the standard time meridian LSTD asfollows:Ws=LocT+(LON−LSTD)  (12)LocT is determined by a function of the local time expressed in hours,as set forth in the following equation:LocT=(Hrs−12)·(−15)  (13)The solar declination d can be expressed as a function of the day of theyear n, as follows: $\begin{matrix}{d = {23.45 \cdot {\sin\left( {\frac{284 + n}{365} \cdot 360} \right)}}} & (14)\end{matrix}$The local time and hours in the day of the year n would be bothavailable within the relay as THR and DDOY. The conductor latitude LATand longitude LON, along with the standard time meridian LSTD, must beentered as settings.

Another determination which must be made is the conductor thermalresistance TRA, which is determined by Equation 15 below:$\begin{matrix}{{TRA} = \frac{{TC} - {TA}}{Q_{radiation} + Q_{convected}}} & (15)\end{matrix}$and is equal to the temperature difference between the conductor and thesurrounding air, so that the heat transferred to the conductor is oneunit of power.

The radiated heat losses can be computed as follows:Q _(rad) =S·E·A·(KC ⁴ −KA ⁴)  (16)with the following variables: $\begin{matrix}{S = {\text{Stephan}\text{-}\text{Boltzman}\quad\text{constant}\quad\left( {W\text{/}{^\circ}\quad k^{4}{ft}^{2}} \right)}} \\{= {0.527e\text{-}8}} \\{E = {{thermal}\quad{emissivity}\quad{constant}\quad({unitless})}} \\{= {0.23\quad{for}\quad{new}\quad{conductor}}} \\{= {0.91\quad{for}\quad{blackend}\quad{conductor}}} \\{A = {{area}\quad{of}\quad{circumscribing}\quad{cylinder}\quad\left( {ft}^{2} \right)}} \\{{KC} = {{conductor}\quad{temperature}\quad\left( {{^\circ}\quad k} \right)}} \\{{KA} = {{ambient}\quad{temperature}\quad\left( {{^\circ}\quad k} \right)}}\end{matrix}$After adjusting the units, $\begin{matrix}{Q_{rad} = {0.138 \cdot {DIA} \cdot E \cdot \left\lbrack {\left( \frac{KC}{100} \right)^{4} - \left( \frac{KA}{100} \right)^{4}} \right\rbrack}} & (17)\end{matrix}$where DIA (the conductor diameter) is in inches and the units of Q_(rad)are in kW/kft.

The value of the convected heat losses (Q_(convected)) is determined bythe following equation: $\begin{matrix}{Q_{conv} = {\left\lbrack {1.01 + {0.371\left( \frac{{DIA}\quad\bullet\quad R\quad\bullet\quad V}{H} \right)^{0.52}}} \right\rbrack\quad\bullet\quad K\quad\bullet\quad\left( {{TC} - {TA}} \right)}} & (18)\end{matrix}$where the variables are:

-   -   R=air density (lb/ft³)    -   V=air velocity (ft/hr)    -   H=absolute viscosity (lb/hr ft)    -   K=thermal conductivity (W/ft²° C.)

The value of thermal capacity (THC) is determined by multiplying thenumber of pounds of aluminum (WA) by the aluminum specific-heat andadding the number of pounds of steel (WS) multiplied by the steelspecific heat, as in the following equation: $\begin{matrix}{{THC} = \frac{{{WA}\quad\bullet\quad 428.8} + {{WS}\quad\bullet\quad 204.9}}{100}} & (19)\end{matrix}$

The above equations, as previously indicated, used to determine athermal model, are known. However, previously a manufacturer of aprotective relay, using such a thermal model for temperaturedetermination of conductors, would directly provide all the specificvariables at the factory and completely set up the thermal model in therelay for the customer. The customer thus has no ability to affect thethermal model in this arrangement. Frequently, however, this would beless than optimum, since the customer's use/location would likelyincrease the accuracy of the thermal model.

In the present invention, a protective relay includes a logic capabilityby which the user can directly program the thermal model equations inthe relay. In order to provide the information necessary fordetermination of the thermal model, the following settings are made bythe user. The first group of settings is the solar model settings.

Solar Model Settings

-   -   DSH=Default solar heating (kW/kft)    -   SAC=Solar absorption coefficient (unitless)    -   DIA=Conductor diameter (in)    -   LSTD=Longitude of time standard (degrees west longitude)    -   LON=Longitude of conductor (degrees west longitude)    -   LAT=Latitude of conductor (degrees north latitude)        The diameter of the conductor is corrected, both according to        elevation and atmosphere, in accordance with the following        equation:        DIA _(cor) =k ₁ ·k ₂ ·DIA  (20)

Values of k₁ and k₂ are shown in Tables I and II provided below. TABLE IElevation (ft) k1 0 1.00 5000 1.15 10000 1.25 15000 1.30

TABLE II Atmosphere k2 Clean 1.00 Industrial 0.82

The longitude of the time standard is entered according to Table III.TABLE III Time Zone Meridian Eastern  75° W Central  90° W Mountain 105°W Pacific 120° W

Next, the thermal model settings as set forth below must be entered.

-   -   rac=AC resistance at 25° C. (Ω/k)    -   rdelt=temperature coefficient of AC resistance (Ω/° C. kft)    -   THC=thermal heat capacity (kJ/° C. kft)    -   TRA=thermal resistance to ambient (° C. kft/kW)

Next, the temperature settings must be entered. The estimated offsettemperature EOT is the temperature difference between the ambienttemperature available and the estimated hottest ambient temperaturealong the line. It must be entered as a number other than zero if it isestimated that a difference in fact exists. If not, it is entered atzero.

-   -   EAT=estimated ambient temperature (° C.)    -   EOT=estimated offset temperature (° C.)    -   TTH=high temperature threshold (° C.)    -   TTL=low temperature threshold (° C.)    -   TI=conductor initial temperature (° C.)

Lastly, the following logic switches must be set.

-   -   THE=thermal trip enable (1/O.)    -   TSE=thermal sensor enable (1/O)    -   SGE=solar generator enable (1/O)        THE has to be set to 1 if the line is to be tripped due to        excessive temperature; TSE is set to 1 if an external sensor is        available for the ambient temperature. SGE is set to 1 if        Q_(sun) is computed from Equation (9). It is set to zero if        Q_(sun) is introduced as equal to DSH.

One example of a particular implementation is set forth below. A lineconductor is made of Drake cable type with the following basiccharacteristics that can be extracted from tables: Diameter DIA = 1.108in (k1 = k1 = 1.0) Thermal emissivity constant E = 0.5 (unitless) ACresistance per mile at 25° C. RL = 0.117 Ω/mile AC resistance per mileat 75° C. RH = 0.139 Ω/mile Weight of aluminum 1000 ft WA = 750 lb/kftof conductor Weight of steel of 1000 ft WS-344 lb/kft of conductorThe value of rac, the conductor resistance per 1000 ft, is determinedfrom the value of RL: $\begin{matrix}{{rac} = {\frac{RL}{5.28} = {\frac{0.117}{5.28} = {0.022159\quad\Omega\text{/kft}}}}} & (21)\end{matrix}$The value of rdelt, the conductor temperature coefficient, is computedfrom: $\begin{matrix}\begin{matrix}{{rdelt} = \frac{{RH} - {RL}}{\left( {75 - 25} \right)\quad\bullet\quad 5.28}} \\{= \frac{0.139 - 0.117}{50\quad\bullet\quad 5.28}} \\{= {0.0000833\quad\Omega\text{/}{^\circ}\quad\text{Ckft}}}\end{matrix} & (22)\end{matrix}$The thermal heat capacity is determined by multiplying the number ofpounds of aluminum WA by the aluminum specific heat and adding thenumber of pounds of steel multiplied by the steel specific heat:$\begin{matrix}{{THC} = {\frac{{{WA}\quad\bullet\quad 428.8} + {{WS}\quad\bullet\quad 204.9}}{1000} = {392.086\quad\text{kJ/}{^\circ}\quad\text{Ckft}}}} & (23)\end{matrix}$TRA is computed using equations 15 through 18 and using a conductortemperature of 90° C. and an ambient temperature of 40° C.

-   -   First, we compute the radiated power. From Equation 17, we have:        $\begin{matrix}        {Q_{rad} = {0.138\quad\bullet\quad E\quad\bullet\quad{{DIA}\left\lbrack {\left( \frac{KC}{100} \right)^{4} - \left( \frac{KA}{100} \right)^{4}} \right\rbrack}\quad{or}}} & (24) \\        \begin{matrix}        {Q_{rad} = {0.138\quad\bullet\quad 0.5\quad\bullet\quad 1.108\quad{\bullet\quad\left\lbrack {\left( \frac{363}{100} \right)^{4} - \left( \frac{313}{100} \right)^{4}} \right\rbrack}}} \\        {= {5.937\quad{kW}\text{/kft}}}        \end{matrix} & (25)        \end{matrix}$        The convected power is computed from Equation 18 with the        following assumed air constants:    -   R=Air density=0.0752 lb/ft³(20° C., sea level)    -   V=Air velocity=2 ft/sec=7200 ft/hr    -   H=Absolute viscosity−0.0439 lb/hr ft (20° C.)    -   K=Thermal conductivity=0.00784 watts/ft²° C.(20° C.)        $\begin{matrix}        \begin{matrix}        {Q_{conv} = {\left\lbrack {1.01 + {0.371\left( \frac{1.108\quad\bullet\quad 0.0752\quad\bullet\quad 7200}{0.0439} \right)^{0.52}}} \right\rbrack\quad\bullet}} \\        {0.00784\quad\bullet\quad\left( {90 - 40} \right)} \\        {= {20.964\quad{kW}\text{/kft}}}        \end{matrix} & (26)        \end{matrix}$        Finally, we have for TRA: $\begin{matrix}        {{TRA} = {\frac{{TC} - {TA}}{Q_{rad} + Q_{conv}} = {\frac{90 - 40}{5.937 + 20.964} = {1.859{^\circ}\quad\text{Ckft/}{kW}}}}} & (27)        \end{matrix}$        The time constant of the first order thermal model is provided        then as:        Thermal time constant=THC·TRA=392.086·1.859=728.9 s=12.15 nm        (minutes)  (28)        The practical meaning of this time constant is that if a step        current is applied to the conductor, it will take 12.15 minutes        for the conductor to reach 63% of the new steady-state        temperature value.

Assume that the line is located in the Montreal (Canada) area. Also,assume the routine is started with the line open in summer time. Thefinal set of settings that has to be introduced into the routine asconstants as follows: DSH default solar heating = 0 SAC solar absorptioncoefficient = 0.5 DIA conductor diameter = 1.108 LSTD longitude of timestandard = 75.00 LON longitude of conductor = 73.43 LAT latitude ofconductor = 45.56 rac AC resistance at 25° C. = 0.022159 rdelt temp.coeff. Of AC resistance = 0.0000833 THC thermal heat capacity = 392.086TRA thermal resistance to ambient = 1.859 EAT estimated ambienttemperature = 25 EOT estimated offset temperature = 0 TH hightemperature threshold = 90 TL low temperature threshold = 80 TIconductor initial temperature = 25 THE thermal trip enable = 0 TSEthermal sensor enable = 1 SGE solar generator enable = 1

FIG. 2 shows a block diagram for the above example to produce an alarmor a trip signal. The block diagram includes a thermal trip enablesignal 25 to an AND gate 26 (with the output from the trip comparator).The output of AND gate 26 is applied to a timer 27, while the output ofthe alarm comparator 22 is applied to a timer 29. The timers 27 and 29are conditioning timers with pick-up and drop-out times of 10 powersignal cycles.

As indicated above, settings entered by the user, such as in the exampleabove, are implemented into the thermal model by a logic program whichis present in the relay. The requirements of one such logic program asan example are set forth below. The program flow chart for programmingthe relay to implement the user-defined thermal model in the relay isshown in FIG. 3.

At the start of the program sequence, it is first determined whether ornot it is the first processing sequence for the thermal model, as shownat block 30. If yes, the initial conductor temperature is provided, atblock 32. If not, then if SGE (solar generator enable) is equal to one,as determined at block 34, then Q₂, is computed at block 36, while ifnot, it is set to the DSH (default) value, at block 38. The largest ofthe three phase currents (A, B, C phase) is then chosen as the conductorcurrent at block 40. The temperature is then calculated using thethermal model, at blocks 42 and 44. If the temperature is larger thanthe THL value (trip), as determined at block 45, then the circuitbreaker on the line is tripped. If the temperature value is not equal toor above the trip threshold value but above the alarm value, asdetermined at block 48, then an alarm bit is set at block 50. Thesequence is then ended. The sequence repeats itself at specificintervals.

As indicated above, applicants' invention includes the use of logiccircuits and logic equations in the relay to accomplish a userprogrammable thermal model. In addition to the functional flow chartdiscussed above, one example of the logic coding to provide theprogrammable capability is shown in FIG. 4. In this example, lines 1-19introduce the power line thermal element settings, lines 20-39 providethe computation of Q_(sun), lines 40-42 provide the conductor initialtemperature value, lines 43-46 determine the maximum phase currentdetermination, lines 47-48 establish the ambient temperature, lines49-52 establish the conductor temperature, lines 53-55 determine thealarm output and lines 56-58 determine the trip output.

The combination of the relay having the structure and capability ofpermitting the user to enter appropriate settings for a thermal model,along with the logic capability to implement those settings into thethermal model and then accomplish the calculation in accordance with thestored thermal model equations, results in a user-programmable thermalmodel system for protective relays.

Although a preferred embodiment of the invention has been described forpurposes of illustration, it should be understood that various changes,modifications and substitutions may be incorporated in the embodimentwithout departing from the spirit of the invention which is defined inthe claims which follow.

1. A protective relay system for power line thermal protection usingprogrammable logic present within the relay with the capability ofconstructing associated logic equations, comprising: a protective relayfor power lines, which includes a programmable logic capability by whichthe end user of the protective relay can enter settings which are thenused by the relay in carrying out its thermal protection functions; aset of stored thermal model equations which when solved emulate thetemperature of a power line conductor, based on a plurality ofindividual setting values which are enterable into the relay by the enduser, and wherein the logic and logic equations implement the enteredsetting values into the thermal model equations which produce anemulated temperature of the conductor; and means for providing anindication when the temperature of the conductor exceeds a preselectedvalue.
 2. The system of claim 1, wherein the settings entered by the enduser include solar model settings directed toward heating of theconductor affected by solar considerations, thermal model settings whichare determined from physical aspects of the conductor and temperaturesettings.
 3. The system of claim 2, wherein the solar model settingsinclude a default solar heating value, a solar absorption coefficient,the conductor diameter, the longitude of time standard, the longitude ofthe conductor and the latitude of the conductor, wherein the thermalmodel settings for the conductor include its AC resistance at 25° C.,the temperature coefficient of the AC resistance, the thermal heatingcapacity and the thermal resistance to the ambient temperature, andwherein the temperature settings include the estimated ambienttemperature, the estimated offset temperature, the high temperaturethreshold, the low temperature threshold and the conductor initialtemperature.
 4. The system of claim 1, including a first comparatorwhich compares the temperature from the logic output with a firsttemperature threshold, and wherein if the temperature at the logicoutput exceeds the first threshold, an alarm is provided, and includingfurther a second comparator which compares the temperature at the logicoutput with a second threshold, wherein if the second threshold isexceeded, a trip signal for a circuit breaker is provided.
 5. The systemof claim 1, wherein the conductor temperature is expressed as a firstorder differential equation in accordance with${{P - L} = {{THC}\frac{\mathbb{d}{TC}}{\mathbb{d}t}}},$ where P isequal to the heat power supplied to the conductor, L is the conductorheat losses, THC is the conductor heat thermal capacity and TC is theestimated conductor temperature.
 6. The system of claim 5, wherein theheat power supplied to the conductor=I²·(rac)+(TC−25)·rdelt)+Q _(sun),where I is equal to the conductor current, rac is equal to the ACconductor resistance at 25° C., and rdelt is equal to the temperaturecoefficient of the AC resistance, Q_(sun) is equal to the heat powerinput from the sun, and wherein the conductor heat losses areQ_(radiated)+Q_(convected), the radiated heat losses and the convectionheat losses.
 7. The system of claim 1, wherein the relay is a distancerelay.